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211.
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213.
The magnetic chemically peculiar (CP2) stars of the upper main sequence are well‐suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as α2 Canum Venaticorum (ACV) variables. We have analysed around 3850000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least‐squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Strömgren uvbyβ measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astro‐physical parameters for the investigated objects. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
214.
V. P. Arkhipova V. F. Esipov N. P. Ikonnikova G. V. Komissarova 《Astronomy Letters》2008,34(7):474-485
We have constructed the light curves of the symbiotic nova V1016 Cyg for the period 1971–2007 in a homogeneous photometric system close to UBV using our observations with the Zeiss-600 SAI telescope. Based on the observational data obtained with the 125-cm SAI telescope in 2000–2007, we have performed absolute spectrophotometry of the star in the range λ3700–9300 Å. The derived line intensities are compared with the data of other authors in the preceding years (1965–1988). The behavior of nebular lines showed the variations in electron density and, probably, electron temperature in the [OIII] emission region caused by a variable stellar wind from the hot component. All the available observations of the star confirm the theoretical conclusion that the nova-like outburst of V1016 Cyg was produced by a thermonuclear flash in the accreted envelope of a white dwarf. 相似文献
215.
Yu. A. Fadeyev 《Astronomy Letters》2008,34(11):772-784
The evolution of Population I stars with initial masses 70M ⊙ ≤ M ZAMS ≤ 130M ⊙ is considered. The computations were performed under various assumptions about the mass loss rate and were terminated at the phase of gravitational contraction after core helium exhaustion. The mass loss rate at the helium burning phase, ?3α , is shown to be the main parameter that determines the coefficients of the mass—luminosity relation for Wolf—Rayet stars. Several more accurate mass—luminosity relations for mass loss rates ? = f 3α ?3α , where 0.5 ≤ f 3α ≤ 3, are suggested, along with the mass—luminosity relation that combines all of the evolutionary sequences considered. The results of the stellar evolution computations were used as initial conditions in solving the hydrodynamic equations describing the spherically symmetric motions of a self-gravitating gas. The outer layers of massive Population I stars are unstable against radial oscillations throughout the helium burning phase. The oscillation amplitude is largest at enhanced carbon and oxygen abundances in the outer stellar layers, i.e., at a lower initial stellar mass M ZAMS or a lower mass loss rate during the entire preceding evolution. In the course of evolution, the radial oscillation amplitude decreases and the small nonlinearity of the oscillations at M < 10M ⊙ allow the integral of mechanical work W done by an elementary spherical layer of gas in a closed thermodynamic cycle to be calculated with the necessary accuracy. The maximum of the radial dependence of W is shown to be located in layers with a gas temperature T ~ 2 × 105 K, where the oscillations are excited by the iron Z-bump κ-mechanism. Comparison of the radial dependences of the integral of mechanical work W and the amplitude of the radiative flux variations suggests that the nonlinear radial oscillations of more massive Wolf—Rayet stars are also excited by the κ-mechanism. 相似文献
216.
A number of empirical relationships are shown to indicate an increase in the abundance of phosphorus logε(P) with height in the atmosphere of HR 1512. These include: (a) a correlation of logε(P) with the observed equivalent width Wobs of PII lines; (b) a correlation of logε(P) with the wavelength of the lines; (c) a systematic divergence in the values of logε(P) for lines with different excitation potentials El; in particular, lines with lower El correspond on the average to higher abundances logε(P); and, (d) a distinct dependence of logε(P) on the average geometrical height of formation, Hf. In addition, assuming that logε(P) is constant in the star's atmosphere leads to a systematic discrepancy between the theoretical equivalent widths Wth and the observed values Wobs. By a trial and error method we have found a distribution of the phosphorus abundance logε(P) with height H such that the systematic difference between Wth and Wobs vanishes. It turned out, however, that a simpler, step distribution of logε(P) yields equally good agreement between Wth and Wobs. Although the solution is nonunique, both distributions have some features in common, specifically: (1) a sharp rise in logε(P) occurs in the same range of heights H corresponding to optical thicknesses τ
5000 ≈ 10−2–10−3, i.e., stratification of phosphorus takes place in rather high layers of the atmosphere of HR 1512, and (2) the upper bound,
logε
up(P) = 8.9, is the same in both cases, so that in the region of the rise, logε(P) increases by 3.4 dex. A comparison with available data for HgMn, Am, and Ap type stars shows that similar sharp changes in
the abundances logε of several elements occur in other CP stars, at the same optical depths or in even higher layers of their atmospheres.
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Translated from Astrofizika, Vol. 51, No. 2, pp. 239–253 (May 2008). 相似文献
217.
Hideyuki Saio C. Simon Jeffery 《Monthly notices of the Royal Astronomical Society》2000,313(4):671-677
We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium‐burning shell is approximately 0.25 M⊙ , the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models.
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario. 相似文献
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario. 相似文献
218.
The photometric JHKLM observations of the symbiotic novae V1016 Cyg and HM Sge in 1978–1999 are presented. Parameters of the cool stars themselves and the dust envelopes are estimated. The periods of 470±5 days (for V1016 Cyg) and 535±5 days (for HM Sge) are reliably determined from the entire set of our photometric J data for V1016 Cyg and HM Sge. In addition, monotonic light and color variations are observed on a time scale of several thousand days, with the increase in infrared brightness occurring with the simultaneous decrease in infrared color indices; i.e., the dust envelopes in which both components of the systems were embedded before the outburst of their hot sources in 1964 and 1975, respectively, had continued to disperse until late 1999. The amplitudes of these variations for HM Sge are almost twice those for V1016 Cyg. For HM Sge, the dust envelope reached a maximum density near JD 2447500 and then began to disperse. In the case of V1016 Cyg, a maximum density of the dust envelope was probably reached near JD 2444800, and its dispersal has been continuing for about 20 years. Thus, in both symbiotic novae, their dust envelopes reached a maximum density approximately eight years after the outburst of the hot component and then began to disperse. An analysis of the color-magnitude (J–K, J) diagram reveals that grains in the dust envelopes of V1016 Cyg and HM Sge are similar in their optical properties to impure silicates. The observed [J–K, K–L] color variations for the symbiotic novae under study can be explained in terms of the simple model we chose by variations in the Mira's photospheric temperature from 2400 to 3000 K and in the dust-envelope optical depth from 1 to 3 at a wavelength of 1.25 µm for a constant grain temperature. The observed J–K and K–L color indices for both symbiotic novae, while decreasing, tend to the values typical of Miras. The dust envelopes of both symbiotic novae are optically thick. The dust envelope around HM Sge is, on the average, twice as dense as that around V1016 Cyg; the Mira in V1016 Cyg is slightly cooler (~2800–2900 K) than that in HM Sge (~2600–2700 K). The dust-envelope density decreases as the Mira's temperature increases. The absolute bolometric magnitudes are $ - 5\mathop .\limits^m 1 \pm 0\mathop .\limits^m 15$ for V 1016 Cyg and $5\mathop .\limits^m 27 \pm 0\mathop .\limits^m 17$ for HM Sge. Their distances are 2.8±0.6 and 1.8±0.4 kpc, respectively; the luminosities and radii of their cool components (Miras) are 8.6×103 L ⊙, 1×104 L ⊙, 500R ⊙, and 540R ⊙. The radii of their dust envelopes are 1400R ⊙ and 1500R ⊙; the masses are (3?3.3) × 10?5M⊙ and (4?8) × 10?5M⊙ The dust envelope of V1016 Cyg disperses slower than that of HM Sge by almost a factor of 25. 相似文献
219.
George Wallerstein † Sean Matt† Guillermo Gonzalez† 《Monthly notices of the Royal Astronomical Society》2000,311(2):414-422
We have used echelle spectra of resolving power 35 000 to derive chemical abundances and the 12 C/13 C ratio in the 1.9-d carbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparison star. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. In addition, C and N are substantially in excess, and a small deficiency in O is present. We interpret these anomalies as resulting from the appearance on the stellar surface of material enriched in 12 C by the 3- α process, followed by CNO cycling to convert 12 C to 13 C and 14 N. In addition, some 16 O has been processed to 14 N. The partial processing of 16 O to 14 N indicates that substantial 17 O may be present. Proton capture seems to have enhanced 23 Na from the Ne isotopes. 相似文献
220.